LibJS/Tests: Use eval() for toEvalTo(), not Function()

Since we have had eval() for a while now, we can finally use it here -
this allows us to get rid of the confusing return statements in tested
source code.

Userland/Libraries/LibJS/Tests/comments-basic.js

@@ -7,7 +7,7 @@ var i = 0;
 i++;
 */
 /**/ i++;
-return i;`;
+i;`;
 
     expect(source).toEvalTo(1);
 });
@@ -22,7 +22,7 @@ i++;
 --> i++;
 /**/ --> i++;
 j --> i++;
-return i;`;
+i;`;
     expect(source).toEvalTo(2);
 });